Franson went undrafted in the 2002 WHL Bantam Draft. Nevertheless, the Vancouver Giants in the Western Hockey League (WHL) offered him a tryout and Franson was successful in making the team.[2] Franson played fulltime for the Giants starting in the 2004–05, where he put up 13 points. Franson then improved to 55- and 51-point campaigns with the Giants in the next two seasons. In 2006, he helped lead the Giants to a President's Cup as WHL champions, earning a berth in the 2006 Memorial Cup. He recorded the only hat-trick of the tournament and led all defencemen in scoring with 5 points in 5 games. In the subsequent off-season, Franson signed an entry-level contract with the Predators in July 2006.[3] The next year, he was named to the WHL West First All-Star Team[4] and was nominated for the Bill Hunter Memorial Trophy as the league's top defenceman (awarded to Kris Russell of the Medicine Hat Tigers).[5] After falling to the Medicine Hat Tigers in the 2007 WHL Final, Franson and the Giants captured the 2007 Memorial Cup as tournament hosts, defeating the Tigers in the final.

On July 3, 2011, Franson and Matthew Lombardi were traded to the Toronto Maple Leafs for Brett Lebda and Robert Slaney.[7] During the following 2011–12 season, Franson played 57 games for the Leafs putting up 21 points and finishing with a +/- of -1.

On October 1, 2012, Franson signed a contract with Brynäs IF of the Swedish Elitserien due to the 2012–13 NHL lockout. On January 13, 2013 he signed a one-year contract with the Toronto Maple Leafs as a restricted free agent.

On May 13, 2013, Franson scored two goals for the Toronto Maple Leafs in Game 7 of the first round of Stanley Cup Playoffs versus the Boston Bruins.

On July 21, 2014, Franson and the Leafs agreed to a 1 year contract worth $3.3 million. At the end of the contract, Franson will become an unrestricted free agent.[8]

On February 15, 2015, Franson was traded to the Nashville Predators along with Mike Santorelli in return for Olli Jokinen, Brendan Leipsic and a first round draft pick.[9] This marked both Santorelli's and Franson's second tenure with Nashville.